Effect of Ethanol on Dendritic Spines During Normal Aging in Rats Purkinje neurons and granule neurons are two major cell populations in the cerebellar cortex. Purkinje neurons are well recognized by the size and complexity of their dentritic arbors. The numerous spines on the arbors serve as postsynaptic sites or reception of afferent input from the parallel fiber axons of granule neurons. The presynaptic axonal membranes, associated vesicles containing neurotransmitters and the thickened postsynaptic spinous membranes from structural complexes called synapses. The total number of synapses on Purkinje dendritic arbors of normally aging rats was known to be significantly reduced after chronic ethanol treatment, an effect that was reversed after recovery from the ethanol treatment. There were no ethanol-induced decreases in the number of parallel fibers because the number of granule neurons did not change. The overall size of the dendritic arbors was also unchanged, but dendritic segments within the arbors were longer than normal following ethanol tre atment, suggesting that some segments and their branch points had been deleted. Delection of segments results indirectly in a loss of spines and of synapses, but direct damaging effects of ethanol on spines would also result in deletion of spine-depleted segments. The purpose of this study was to determine whether the loss of synapses represented a direct effect of ethanol on the density of spines (spines/um) on the dendritic segments. Spine densities on terminal dentrites were determined in Purkinje neurons from 40 old male Fischer 344 rats that were assigned randomly to one of three treatments groups, a chow-fed control group (n=8), an isocaloric and isovolumetric (pair-fed) control group (n=16), or an ethanol-fed group (n=16). Brain tissues were prepared by the Golgi-Cox procedure for light microscopy and coded to avoid investigator bias. 14 Purkinje neurons from each rat (a total of 560 cells) were randomly selected for quantitation of spines. In each neuron, spine counts and terminal segment lengths were determined for 14 paired and 14 umpaired terminal dendritic segments (a total of 15,680 measurements). Group averages were tested by ANOVA showed no statistically significant decreases in spine density on spiny branchlets of Purkinje neurons after chronic ethanol consumption. It was concluded that ethanol did not reduce the density of spi nes on dendritic segments of Purkinje neurons directly. Keywords: Cerebellar Cortex, Ethanol, Golgi Cox, Purkinje Neurons, Synaptic Spines